1. Field of the Invention
This invention relates generally to oscillators and more particularly to an improved Butler oscillator in which a high Q, series resonant circuit, such as a piezoelectric crystal is included in a substantially low, constant impedance path and is driven by a non-distorted waveform to achieve superior frequency stability.
2. Description of the Prior Art
In a Butler oscillator a piezoelectric crystal or other high Q, series resonant circuit is interposed in a feedback loop between the input of a voltage amplification stage capable of providing gain in excess of unity and the output of an impedance matching stage. In transistor circuitry the Butler oscillator is implemented by driving an emitter follower (impedance matching) stage with a common base (voltage amplifier) stage. A crystal is interposed between emitters of the two transistors.
The Butler oscillator utilizes the series resonant frequency rather than the parallel resonant frequency of the crystal or equivalent circuit. From the standpoint of frequency stability, the highest stability is attained by operating the crystal in the series resonant mode, because the crystal is not nearly as susceptible to stray circuit capacitance to alter the effective resonant frequency thereof as it would be if operated in the parallel resonant mode.
Another advantage of the Butler oscillator is that the crystal is included in a relatively low impedance path. Since the series impedance of the crystal is relatively low at the series resonant frequency, the Q is maintained at a relatively high level by operating the crystal in a relatively low impedance path. The effective circuit Q would be lowered by operation of the crystal in a series resonant mode in a relatively high impedance path. Higher frequency stability of the oscillator is attained by operation of the crystal in a circuit of the highest attainable Q, since the higher the Q, the more closely the crystal oscillates relative to its true, natural resonant frequency.
Not only should the impedance of the path in which the crystal or equivalent high Q, series resonant circuit is connected be as low as possible, but this impedance should also remain constant. If the impedance changes then harmonic distortion is generated in the crystal drive circuit which, in turn, contributes to frequency instability. Further, to the extent that these impedances rise above their minimum value, the Q of the crystal circuit is reduced.
For optimum frequency stability the crystal should be driven only by a sine wave, because the presence of distortion comprising any harmonic power content in the drive waveform to the crystal will contribute to instability which is proportional to the degree of distortion in the drive waveform.
In the most conventional prior art Butler oscillator circuits it has been impossible to assure that the crystal is connected in a path having a constant circuit impedance, although circuit improvements have been offered to correct this problem with the trade-off of other, severe design limitations. It has also been impossible to assure that the crystal is driven by a relatively pure sinusoidal waveform. For superior frequency stability owing to circuit design, therefore, it would be advantageous to provide an improved Butler oscillator circuit in which the crystal is connected between constant, relatively low impedances and is driven only by a relatively pure sinusoidal waveform.